In a conventional test system, multiple (up to several thousands) measurement instruments are connected to a controlling computer. The measurement instruments can be of different types (e.g., for measuring digital signal levels, analog voltages or currents, low or high frequencies, or even non-electrical properties like temperatures). Typically more than one instrument of each type is available so that multiple devices under test can be tested at once. The controller often accesses groups (subsets) of these instruments (for example, only those digital measurement instruments that are connected to a specific port or terminal of a device under test).
Such a conventional test system provides a mechanism for setting up groups of instruments, typically a limited number (for example 32). One disadvantage of such a mechanism is that the number of available groups is small compared to the number of subsets that are typically accessed by a test program. For example, some devices may use 100 or 200 subsets, with only 32 groups available. Another disadvantage is that setting up or changing the instruments contained in a group is a slow operation, typically requiring several microseconds per instrument which is added to or removed from the group.
In the following, a concept of such a conventional test system and the disadvantages thereof will be described in short. To simplify the exposition, the test system is assumed to contain a number of identical test resources, each of which is connected to one signal pin of one of a number of devices under test.
A mechanism for grouping channels (“common access”) in such a conventional test system uses two 32-bit registers and some internal logic. The mechanism for grouping channels in the conventional test system is shown in a block schematic diagram in FIG. 10.
A first register (a channel code register 11) holds the bitmap of groups to which the channel belongs. Each bit in this register corresponds to one available group. Each test resource of the conventional test system typically comprises its own channel code register 11. The channel code register 11 is typically reprogrammable during a test. As mentioned before this reprogramming is very time-consuming. The number of reprogramming cycles therefore should be minimized during the test.
A second register (a channel mask register 13) selects which groups are currently active. The channel mask register 13 is typically a global register whose content is distributed to the individual test resources for comparing it with the channel code register 11. The currently active groups change typically very often during a test and therefore the channel mask register 13 also changes very often. Based on the comparison of the current content of the channel mask register 13 with the current content of the channel code register 11, at each test resource a common access “select” signal can be determined, whose state indicates whether or not the test resource belongs to at least one group defined by the channel mask register 13. The common access “select” signal may for example be determined by using 32 2-input AND gates, 15-1 to 15-32 (one AND gate per bit of the channel code register 11 and of the channel mask register 13) and one 32-input OR gate 17 per test resource. By using this concept every bit of the channel mask register 13 is compared with its corresponding bit in the channel code register 11 and if at least one of the comparisons provides a true information, the OR gate 17 also provides a true information as the common access “select” signal, i.e. the test resource is part of at least one group defined in the channel mask register 13.
It is also possible that several groups can be active at once, if more than one bit in the channel mask register 13 is set.
The main disadvantage of a conventional test system which uses the concept shown in FIG. 10 is the limited, small number of groups that can be supported (32 in the example of FIG. 10). This involves frequent reprogramming of the channel code register 11 when more than this small number of groups are needed (for example, in multisite testing), leading to decreased test throughput.